Methanol oxidation on palladium compared to rhodium at ambient pressures as probed by surface-enhanced Raman and mass spectroscopies

The influences of surface speciation upon the catalytic oxidation kinetics of methanol on palladium under ambient-pressure flow-reactor conditions was examined by surface-enhanced Raman spectroscopy (SERS) combined with mass spectrometry (MS) and compared with corresponding data on rhodium. The former technique provides uniquely sensitive surface vibrational information under real-time (approximate to 1 s) in situ conditions, by utilizing ultrathin catalyst films electrodeposited onto an inert SERS-active gold substrate. These transition-metal surfaces exhibit sufficiently robust SERS activity to enable temperature-dependent spectral measurements over the range 25-500 degrees C. Parallel kinetic measurements undertaken with MS show the occurrence of methanol decomposition (to CO and H-2) in the absence of O-2 on both Pd and Rh. While the presence of a molar deficiency of O-2 yields methanol oxidation (to form CO2 and H-2) in addition to methanol decomposition on Rh, only the latter occurred (at slower rates than Ph) on Pd. These dissimilar reaction selectivities are consistent with the absence of surface vibrational features on the latter surface and the observed presence of adsorbed CO on the former. The behavior can be rationalized by the paucity of adsorbed atomic oxygen, O-(ad), On Pd compared with Rh arising from the greater ability of the latter to dissociatively chemisorb O-2. Both catalysts induced exhaustive methanol oxidation (yielding CO2 and H2O) in a heavily O-2-rich reactant mixture, although Pd again yielded less facile reaction kinetics. In addition, a significant catalyst deactivation occurred upon heating Pd in this reactant mixture, which was entirely absent on Rh. The corresponding temperature-dependent SER spectra indicate the formation of palladium oxide (PdO) by 350 degrees C, which was retained entirely upon subsequent cooling. While an oxide (Rh2O3) was also seen by SERS to form on Rh by 350 degrees C under these conditions, this species was removed upon subsequent cooling. Transient SERS measurements following sudden exposure of such oxidized surfaces to a methanol gas stream revealed that PdO was entirely unreactive toward methanol even at 350 degrees C, while, in contrast, Rh2O3 was removed entirely within ca. 5 s. This remarkable difference in oxide reactivity, which accounts for the Pd catalyst deactivation, was deduced to be due primarily to the inability of methanol to yield a suitable adsorbed "oxygen scavenger" by dissociative chemisorption on Pd. The possible involvement of a methoxy intermediate in the reaction on Pd under O-2-rich conditions, as suggested by the SERS data, is also discussed. (C) 1998 Academic Press.